1. Field of the Invention
The invention related to a control circuit of synchronous rectifying with charge pump to improve efficiency of power converters.
2. Description of the Related Art
Synchronous rectifying technologies had been disclosed in many prior arts such as, U.S. Pat. No. 6,995,991 titled “PWM controller for synchronous rectifier of flyback power converter”; U.S. Pat. No. 7,440,298 titled “Synchronous rectification circuit for power converters”; and U.S. Pat. No. 8,072,787 titled “Synchronous rectifying for soft switching power converters”.
FIG. 1 shows a prior art of a power converter with synchronous rectifying. A transistor 20, controlled by a switching signal SW, is coupled to switch a transformer 10 for transferring energy from an input voltage VIN to an output voltage VO of the power converter. When a rectifier 35 (or the body diode of a transistor 30) is turned on to deliver the power from the transformer 10 to an output capacitor 40, the transistor 30 will be turned on to reduce conduction loss of the rectifier 35 (the forward voltage drop of the rectifier 35). A terminal DET of a synchronous rectifying control circuit 50 is coupled to the transistor 30 and/or the transformer 10 for detecting a signal SDET and achieving the synchronous rectifying. The synchronous rectifying control circuit 50 generates a control signal VG at its terminal VG in accordance with the signal SDET. The control signal VG is coupled to switch the transistor 30. In the most of the applications, the power source (VCC) at a terminal VCC of the synchronous rectifying control circuit 50 is supplied by the output voltage VO of the power converter. The drawback of these applications is that the voltage level of the control signal VG cannot sufficiently drive the transistor 30 when the output voltage VO becomes to a low voltage.
FIG. 2 shows a voltage-to-current curve (the output voltage VO versus an output current IO) of the power converter. The output voltage VO would be a low voltage when the power converter is operated in constant current mode. The power source (VCC) of the synchronous rectifying control circuit 50 would be too low to let the control signal VG fully turn on the transistor 30 if the power converter is operated in a region 65, in which the output voltage VO is relatively low. This will cause low efficiency problem to the power converter.